The field of the invention is charged particle beam systems.
Particle beam systems ordinarily contain at least one focus coil having a relatively quick response time for making adjustments in the focus plane of the beam.
A common problem with such coils is that, when the current changes in order to adjust the focus, the temperature changes. That, in turn, changes the permeability of the magnetic material in the lens being adjusted and also changes the dimensions of parts in thermal contact with the focus coil.
The invention relates to a focus coil mounted in proximity to a lens such that the focus coil adds or subtracts from the field of the lens and that provides a set of focus values while consuming the same power for each such value.
A feature of the invention is operation in bipolar modexe2x80x94the coil is driven with current of the same magnitude but opposite sense, thereby consuming the same power while altering the focal plane of the system.
A feature of the invention is the ability to select dynamically a number of focal planes using magnetic coils without disturbing the thermal equilibrium of the system.
Another feature of the invention is the provision of a set of coils, each having the same total number of turns, but having selected numbers of contributing turns that contribute to the magnetic field, the remainder of the total number of turns being arranged in canceling pairs that produce no net magnetic field.
Another feature of the invention is the ability to select a subset of two or three coils (out of the total number of coils) to be energized simultaneously, the various coils combining to provide a range of focus values, with the same number of coils being energized for each focus value.